The present invention relates to a method of producing an aluminum primary base metal by electrolysis.
An aluminum base metal has been principally produced by Hall-Heroult electrolysis. In Hall-Heroult electrolysis, alumina, that is aluminum oxide, and a carbon material for an anode are used as a main raw material and an additive raw material, respectively.
Alumina is usually prepared from alumina-containing ore such as bauxite by alkali extracting and calcining, and is supplied as powder to an electrolysis cell. Alumina prepared in such a manner as explained above usually has a purity of about 98.5 wt %. The alumina has a moisture content and contains from several tens to several hundred of ppm each of metal oxides such as Fe, Si, Ga, V and Ti as shown in Table 1.
The carbon material for anode used as an additive raw material is prepared by mixing calcined coke and a binder in a predetermined proportion and compacting the mixture into briquettes, and is supplied to the top of the anode of an electrolysis cell. Moreover, these materials are sometimes compacted and fired in advance, and set in the electrolysis cell. The carbon material for anode is consumed as the electrolytic reduction of alumina (aluminum oxide) proceeds. The carbon material used in the anode is a mixture of coke and pitch, and contains about several hundred ppm each of oxides such as Fe, Si, V and Ti. This is because the ordinary purity of coke and pitch is as shown in Table 2.
Although the impurities contained in the alumina (main raw material) and the carbon material for anode (additive raw material) are partly removed during electrolysis, a significant amount is transferred to the product. As a result, the maximum purity of primary aluminum obtained by electrolysis is 99.9 wt % (hereinafter referred to as 3N).
In the present specification, the purity of an aluminum base metal is defined as a value obtained by subtracting the total content of the main impurity elements of Si, Fe, Cu, Ni, Ti, Mn, V, Sn, Zn, Cr, Pb, Zr, Bi and Ga (14 elements) from 100 wt %.
On the other hand, in the field of electrolytic capacitors, magnetic discs and the like where demand for highly pure aluminum has been growing in recent years, aluminum having a purity of about 3N cannot meet the requirements for the properties of the capacitors, discs and the like; demand for highly pure aluminum having a purity of at least 99.95% (hereinafter referred to as 3N5) has been growing.
In order to surely meet the quality requirements explained above, the purity of the aluminum base metal has heretofore been improved by a secondary refining step, by the three layer electrolysis and by the segregation process. However, since the improvement requires a secondary refining step, the production cost rises, and the production efficiency declines.
An object of the present invention is to solve the problems related to the conventional technologies described above, and to provide a method of stably producing an aluminum primary base metal having a purity of at least 99.95 wt % (3N5) by electrolysis.
In order to achieve the object described above, a first invention of the present invention provides a method of producing a highly pure aluminum primary base metal, the method comprising placing, as a main raw material, alumina, the Si component of which has been decreased by acid cleaning, in a Hall-Heroult electrolysis cell.
In the acid cleaning, an aqueous solution of sulfuric acid, hydrofluoric acid, or sulfuric acid plus hydrofluoric acid etc. is used, and an acidic aqueous solution heated at temperature of at least 40xc2x0 C. is particularly preferred from the standpoint of removing Si.
According to a second invention of the present invention, the object explained above is also achieved by a method, of producing an aluminum primary base metal, which comprises preparing an electrolysis anode by using deashed coke and/or pitch as a carbon material for the anode, and charging the electrolysis anode into a Hall-Heroult electrolysis cell as an additive raw material.
As a result of using the production methods of the first and the second invention in combination, a highly pure aluminum in which the impurity contents including the Si and the Fe content are further decreased can be produced.